The invention relates to devices adapted to detect magnetic flux variations so that passage through a given threshold by the amplitude of such a variation per unit time is manifested by the movement of an element adapted to trigger an action such as a signal, an alarm or a safety operation, this element being notably the movable contact of an electrical switch.
It relates more particularly, among such devices, to those which comprise an almost closed electric circuit which can be influenced by the flux variation to be detected and adapted to convert the latter into an induced electromotive force.
If the flux variation concerned is produced by a single and non-repetitive phenomenon, such as, for example, the translation of a magnet or the sudden variation of a direct current, the value of the induced electromotive force is directly proportional to the amplitude of the phenomenon to be detected.
However, if the phenomenon to be detected is an alternating current of frequency f, and then the flux concerned is a sinusoidal flux of the form .PHI.=.PHI..sub.o sin 2 .pi. ft generated along the axis of a winding by circulation of said alternating current in this winding, the inductive electromotive force which is generated, at the terminals of another winding traversed by the same flux, by the variation of this flux in time, is of the form d.PHI./dt= 2.pi. f .PHI..sub.o cos 2.pi. ft: the average value of this electromotive force is zero, but not that of the corresponding rectified voltage, which value is proportional to the product 2.pi. f .PHI..sub.o.
It is such a value which is exploited according to the invention, but in general the rectification concerned is only useful to give a certain direction to the elaborated signal, since this signal is elaborated from the first alternation of the alternating phenomenon to be detected, this first alternation bringing into play by itself a flux variation sufficiently rapid and intense for detection.
Among the devices of the type concerned, the invention relates among others to circuit breakers or safety equipment mounted in the supply of an electrical circuit and designed to cut automatically or "cut-off" this supply from the appearance of a leakage current and/or of an overintensity at any point of said circuit.
It is aimed notably, among such circuit breakers, at those comprising a transformer mounted in the supply leads of the circuit to be protected, which transformer is arranged so as to generate automatically in another lead a variable voltage V when a current unbalance occurs in the first leads and means for exploiting said voltage V so as to cut automatically one at least of said first leads as soon as the amplitude of said voltage exceeds a predetermined threshold.
Such circuit breakers are particularly advantageous for the detection of a leakage current corresponding to a portion of a supply current, which portion, coming from the source to the level of the receiver by following one of the two leads of a supply cord, returns to ground and not to the second lead of said cord through a human body placed involuntarily in contact with a bare conductive portion of the circuit concerned, which creates a particularly serious risk of electrocution in a moist environment.
It is a particular object of the invention to render devices of the type concerned such that they respond better to the various exigencies of practice than hitherto, notably so that their operation is certain and autonomous, that is to say does not necessitate any source of external power for their operation and does not consume any electrical current in the surveillance or vigilant state, and so that they are easy to reset after use.